Transplant associated vasculopathy (TAV) is an accelerated form of atherosclerosis resulting in chronic rejection of vascularized organ grafts and the major cause of graft failure since the advent of novel immunosuppressive regimen. The causes of chronic rejection and TAV are likely multifactorial including immunologic and non- immunologic factors that integrate at the level of the vascular wall leading to a phenotypic switch of endothelial (EC) and smooth muscle cells (SMC), underscoring the development of TAV lesions. Activated EC promote inflammation, coagulation, platelet aggregation and cellular invasion and may lead to EC apoptosis exposing the subendothelial matrix and further promoting thrombosis. In addition, production of cytokines and chemokines by activated EC and invasive leukocytes as well as exposure of the subendothelial matrix provoke a switch in SMC phenotype. Activated SMC (i) promote inflammation and increased synthesis of extra-cellular matrix;(ii) exhibit aberrant proliferation and migration within the neointima and (iii) demonstrate deregulated apoptosis, all culminating in the occlusive vasculopathy of chronic rejection. The pathophysiology of TAV has been recently revisited by the demonstration that injury to the vessel wall of the graft triggers homing of circulating SMC and possibly EC progenitors from the recipient. Understanding the molecular basis for, and preventing the acquisition of, this "pro-atherogenic" EC/SMC phenotype as well as halting the homing of circulating SMC to the neointima are critical for devising new therapeutic approaches for the prevention and treatment of TAV. We have shown that A20 is part of the regulatory cytoprotective response of EC to injury. In EC, in vitro, A20 has a dual anti-apoptotic and anti-inflammatory function supporting its atheroprotective potential. We also demonstrate that A20 is part of the physiologic response of SMC to injury. Overexpression of A20 in SMC inhibits the induction of NF-KB dependent genes implicated in TAV, blocks SMC proliferation and unexpectedly sensitizes neointimal SMC to apoptosis. In vivo, A20 is expressed in EC and SMC of long term surviving rat kidney allografts and is associated with the absence of TAV. In addition, overexpression of A20 in rat carotid artery SMC following balloon angioplasty prevents but also cures neointima formation and promotes healing by increasing re-endothelialization. Based on these findings, we hypothesize that expression of A20 in EC and SMC may beneficially impact TAV. Our aims are to (i) Determine the function of A20 in EC as it pertains to activation, apoptosis and vascular remodeling;(ii) Determine the effect of A20 upon SMC activation, proliferation, phenotype and response to apoptotic stimuli;both aims will include loss and gain of function studies;(iii) Establish a structure/function analysis for A20 in EC and SMC;*[(iv) Evaluate the protective function of A20 against TAV using a mouse model of aortic transplantation. In vivo studies will include gain of function experiments using rAd. mediated transfer of A20 to the aortic graft or alternatively, transgenic mice expressing A20 in their vasculature. They will also include loss of function experiments using aortic allografts from A20 knock-out mice]*.